When a Japanese pet is ceremonially cremated, the owners return after the ashes have cooled to sift through them with special chopsticks, picking out the bones one by one and transferring them to their final resting urn. This was the strangest thing I knew about Japanese funeral rites until I discovered that Buddhist priests there now hold services for robot pets as well.

Between 1999 and 2006, the Sony corporation made and sold 150,000 small robot dogs, known as Aibos, which animated at the joints, and were equipped with a microphone and speakers so that they could respond to simple commands. Unlike real dogs, they required no exercise and no food, and they never excreted either. They did, however, mime peeing, lifting their legs and producing what one owner described as “an indescribably beautiful tinkling sound”.

But what really made Aibos resemble real dogs was not their shape or their behaviour. It was that their owners loved them.

When Sony discontinued the product line, the dogs already made continued to work, and to be loved, and the company would replace the parts that wore out until as late as last year. One company, made from former Sony engineers, specialised in reviving defunct robot pets. Then the supply failed. Dogs that stopped working would never start again.

Buddhist priests, who already perform most of the pet cremations in Japan, are now combining two existing traditions – the love with which pets are treated as family members, and the newer habits of mourning toys or dolls when their time is over. A photo essay shows the result: elderly Japanese holding their defunct Aibos in a line, and the later shelving of the bodies, each labelled carefully with their name and place of origin. A prayer is also said, to release the spirit from its metal carcass. After that, the Aibos can become “organ donors” and be plundered for spare parts.

This isn’t just a story about Japan. It’s really about the question of what makes things alive to us. The answer is surely that anything that can die seems alive, and anything that seems alive will sometime die. One way of asking what is going on is to ask why exactly we are so sure that the robot dogs never were alive. It can’t be purely because they are robots, working according to mechanical and chemical laws. So are we, so far as we can tell. Certainly, the “mentalistic behaviourism” espoused by Dan Dennett holds that life is no more than mechanism of a particular sort, which we, ourselves mechanisms, interpret as something more: thought is merely behaviour that we think is thought. So, if computers can some day be intelligent, and appear to us as persons, why shouldn’t robot dogs?

It is true that an Aibo has a behavioural repertoire more limited than that of even the most stupid dog. But plenty of living things have even less of a range of behaviour. We don’t doubt that slime moulds or nematode worms are alive, even though they do very little with their lives.

Presumably, the real difference is concealed by the indescribably beautiful tinkling sounds: an Aibo does not have any physical transactions with the universe. It does not feed itself or reproduce. It is possible to imagine a robot doing all of these things, or their analogues, but Aibos do not.

They do, however, die. They are, as the funerals make quite clear, entirely loved by some of their owners at least. In this their nearest analogue is actually the Velveteen Rabbit in a wonderful children’s classic – or the altogether grimmer, darker and more realistic version of Russell Hoban’s Mouse and His Child. Both stories deal with children’s toys who come alive because they are loved and which can properly be mourned as a result.

The funerals of the robot dogs are in this sense a perfectly religious act. The priests don’t promise that the Aibos will go to heaven, any more than they do for real pets. But the ceremonial gives shape and habitation to a grief. We’re seldom more completely human than when we mourn things that could never mourn us in return.